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Matrix Effects in Auger neutralization in LEIS

Project leader: Peter Bauer

Low-Energy Ion Scattering (LEIS) is a widely used tool for quantitative surface composition analysis, representing a non-destructive method for characterization of the outermost atomic layers of many different materials ranging from metals to porous insulators and even biological tissues. It is an empirical fact that quantitative analysis is possible, even though the neutralization processes of ions on surfaces are still not sufficiently well understood to give an explanation for this finding. Recent results for neutralization of He+ ions at single crystal surfaces of Cu and Au in the regime of Auger neutralization show that the neutralization probability strongly depends on the orientation of the surface. In the reionization regime it has been found that – due to contributions from deeper layers - the ion yield in a random direction is much higher than in double alignment geometry, where the information depth is limited to the outermost surface layer. This implies that in many standard applications the surface sensitivity is not limited to the outermost atomic layer(s). It has impact also on surface analysis by means of LEIS and raises the question by which means it is possible to obtain quantitative information on the outermost surface. In the present TOF-LEIS study it is planned:

(i) to investigate whether it is generally true that there is one mean AN rate A> for a given sample-projectile combination, leading to a physical matrix effect when comparing nonequivalent surfaces, as has been observed for Cu and Au;
(ii) to analyze how the densities of sp- and d-electrons contribute to AN, or in other words to determine how A> correlates with the band structure of the sample, especially with the density of sp- and d-states in real space;
(iii) to learn about the surface sensitivity of LEIS in the AN regime, especially for sample surfaces with low AN efficiency.

The results of the project will make clear:

(i) how the electronic properties of a metal will influence the probability for Auger neutralization (“band structure effects”);
(ii) how the ion fraction amongst the backscattered projectiles depends on the specific orientation of the crystal surface investigated;
(iii) how a LEIS analysis of the surface composition can be quantified, when performed in the Auger neutralization regime.

This project is funded by the Austrian Science Fund under contract number P20831-N20.